This invention relates in general to color cathode ray tubes, and in particular to methods of fabricating an envelope section for such a tube. Of equal significance, the invention is concerned with apparatus for use in such fabrication.
In general, a color selection electrode or "shadow mask" is a device which is disposed adjacent the luminescent phosphor screen that forms the target electrode of a color cathode ray tube (CRT), to control the landing pattern of one or more electron beams as they are swept across the screen. The shadow mask achieves color selection by partially shadowing the surface of the screen from scanning electron beams, permitting access to selected elemental phosphor areas by those beams. The choice of a color selection electrode for use in color television cathode ray tubes is, by and large, a choice between a spherical or biradial electrode and a cylindrical electrode tensed upon a heavy spring frame, both types being supported within the tube envelope. The most common type of color selection electrode used in color television receivers today is the conventional curved type.
In color picture tubes utilizing a conventional shadow mask, there is a tendency on the part of the mask to "dome" (localized buckling) in those areas where a scene characterized by very high brightness is depicted. For example, in a scene where a high concentration of white is presented for an extended period of time, when the beams sweep that area of the screen the current in each beam peaks precipitously with an attendant localized heating of the mask. As a result of such a concentration of heat, that area of the mask expands and displaces itself from its original "cold" position to a position in which it does not effect proper masking of the writing electron beams. As a result, color purity is degraded. Moreover, because of its vulnerability to "doming", a conventional mask cannot accommodate the power density that a "doming-resistant" tensed mask can.
The general practice in cathode ray tubes manufactured for use in color television receivers is to position the mask at an assigned location, relative to the phosphor sreen, by suspending it from three preselected points disposed about the periphery of the tube's face panel. This suspension accomodates overall thermal expansion of the mask by causing the mask to be displaced toward the screen from its original position by provision of bimetallic support springs; however, such provision cannot resolve the above-described localized "doming" problem caused by concentrated heating in localized areas of the mask.
Insofar as the use of a tensioned color selection electrode is concerned, the most common use of such an electrode has been in connection with the cylindrical faceplate CRT produced by one color television manufacturer. In that tube, the color selection electrode comprises a grid formed of a multitude of parallel conductors tensed across a spring frame suspended conventionally within the tube. This grid serves to mask the writing beams so that each falls upon its assigned light emitting phosphor.
The mask supporting frame is mechanically stressed, as by compressing it, prior to attaching the shadow mask thereto. Upon release of the compression force, restoration forces in the frame establish tension in the mask. An advantage of utilizing a tensed mask resides in the fact that the mask, while under tension, will not dome. The mask retains its desired configuration during normal operating conditions.
Under extreme tube operating conditions, however, electron bombardment of a tensed mask of the type adverted to above can cause a series of grids of the mask to relax and cause color impurities. A cathode ray tube utilizing a tensed mask of the type adverted to above is described in U.S. Pat. No. 3,638,063.
The color television cathode ray tube in most common usage today employs a faceplate which approximates a section of a large radius sphere. The shadow mask in such a tube, of course, is contoured to match the faceplate. A trend today is toward a flatter faceplate which, in turn, calls for a flatter shadow mask. However, a flat mask is inherently less mechanically stable than a curved mask. Accordingly, to acquire stability, resort is had to a thicker mask, for example, one having a thickness in the order of 10 to 12 mils. This is approximately twice the thickness of a conventional curved mask. However, when resort is had to a 10 to 12 mil mask the aperture etching process is much more difficult. Specifically, in order to prevent aperture limiting of the beam at the outer reaches of the mask, as would be encountered in a 90 degree tube, the apertures have to be etched at an angle to the plane of the mask, rather than etched substantially perpendicular to that plane as is the case for a conventional curved mask. By way of resolving this aperture etching problem, applicant teaches the use of an envelope section for a color television cathode ray tube characterized by a thin, flat, tensed foil which, by virtue of its mounting, is mechanically stable and which is thin enough as to not be afflicted with the aggravated aperture etching problems posed by a thick mask.